1. Field of the Invention
The present invention relates to switching power supply units, and more particularly, to a self-excited-oscillation resonance-type switching power supply unit that allows for power saving during a light load condition (during standby).
2. Description of the Related Art
A resonance-type switching power supply unit is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-187664.
This power supply unit performs a soft-switching operation, and thus achieves a zero-voltage switching operation by alternately turning on and off two switching elements. This can significantly reduce switching loss, which can achieve high efficiency at rated load. FIG. 10 is a circuit diagram of the power supply unit of the related art.
In this power supply unit, switching elements Q1 and Q2 are turned on at zero voltage and the switching element Q2 is turned off at near zero current, thereby reducing switching loss significantly. A secondary-side rectifying element Ds is turned on at zero current. Concurrently, the waveform of current therein exhibits a relatively sharp rise from zero current, reaches a peak point at which the rate of current change becomes zero, and returns to a point of zero current, thereby turning off the rectifying element Ds. Thus, the waveform of current flowing through the rectifying element Ds becomes a substantially rectangular shape, so that the peak current value can be kept low and the effective value of current can be reduced, thereby reducing conduction loss. Thus, these effects lead to a high efficiency operation.
Meanwhile, a switching power supply unit that is adapted to perform an intermittent oscillation operation at light load is disclosed in Japanese Unexamined Patent Application Publication No. 2000-350449.
This power supply unit is a one-switching-element type switching power supply unit, which is commonly called a xe2x80x9cringing choke converterxe2x80x9d, and can achieve high efficiency by performing an intermittent oscillation at light load. FIG. 13 is a circuit diagram of the power supply unit of the related art.
The above-described power supply units of the related art, however, have the following problems.
FIG. 11 is a waveform diagram illustrating operations, at light load, of the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 11-187664. Referring to FIG. 11, reference symbols S1 and S2 indicate signals representing ON and OFF states of the switching elements Q1 and Q2, respectively; reference symbol Vds1 indicates the waveform of a voltage across a capacitor C1; and reference symbol Id1 indicates the waveform of a current flowing in the switching element Q1.
As shown, when the ON-time of the switching element Q2 is substantially constant regardless of the load condition, energy stored in the transformer is partially regenerated into input power at light load. Thus, this switching power supply unit has a problem in that the circulating current is increased and the efficiency is reduced due to conduction loss. In FIG. 11, shaded areas represent regenerative current.
FIG. 12 is a waveform diagram illustrating operations when the ON-time of the switching element Q2 is designed to be short at light load to decrease the circulating current. However, in this case, while the switching elements Q1 and Q2 perform a zero-voltage switching operation, there is a problem in that the efficiency is reduced, for example, since switching loss is increased in a drive circuit due to an increase in switching frequency.
This power supply unit can control the ON-time of the second switching element to be shorter than the ON-time at rated load (see FIG. 12) to reduce conduction loss at light load. Yet, the timing (turn-off timing) at which the ON-time of the second switching element ends is not set to a timing at which energy release from the secondary winding ends or is earlier than that timing. In other words, even when energy release from the secondary winding ends, the ON-time of the second switching element still continues. As a result, when the second switching element is turned off, current flows through a diode that is connected in parallel with the first switching element. This state is equal to a state in which the first switching element is virtually in a conduction state, which means that the first switching element cannot be turned off. This prevents the operation of the first switching control circuit from proceeding to an intermittent oscillation mode, which will be described later.
FIG. 14 is a waveform diagram illustrating operations, at light load, of the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 2000-350449. Referring to FIG. 14, reference symbol S1 indicates a signal representing ON and OFF states of a switching element Q1, Vds1 indicates the waveform of a voltage across the switching element Q1, and Id1 indicates the waveform of current in the switching element Q1.
As shown, when intermittent oscillation is performed at light load, a large switching surge voltage is generated during intermittent oscillation. As a result, a high voltage switching is required, which can inhibit high efficiency. Additionally, the switching power supply unit has a problem in that an electronic apparatus that is connected thereto malfunctions due to a large output voltage ripple, thus requiring a filter circuit for removing the output voltage ripple.
In addition, the switching power supply unit has difficulty in achieving high efficiency due to switching loss, compared to a soft-switching power supply unit.
In order to overcome the problems described above, preferred embodiments of the present invention provide a self-excited-oscillation switching power supply unit that has a simple configuration with fewer components than conventional devices and that achieves high efficiency at light load, in such a manner that the operational modes are changed by adequately changing, at light load and at rated or heavy load, a time constant for determining the ON-time of ON-time control circuits in switching control circuits for controlling the ON and OFF operations of switching elements, and an intermittent oscillation operation is performed at light load.
According to a preferred embodiment of the present invention, a two-switching-element-type switching power supply unit includes a first switching control circuit and a second switching control circuit which have novel configurations.
Specifically, the second switching control circuit is preferably configured as follows.
The second switching control circuit includes a second On-time control circuit. At light load, the second ON-time control circuit controls the ON-time of a second switching element such that the ON-time of the second switching element becomes shorter that the ON-time at rated load and energy release from a secondary winding is completed after the ON-time of the second switching element ends.
According to a preferred embodiment of the present invention, at light load, the second ON-time control circuit controls the ON-time of the second switching element such that the energy release from the secondary winding is completed after the end of the ON-time of the second switching element. Also, before a voltage generated across the first drive winding causes the first switching element to be turned on, a first impedance circuit causes a first control transistor to go into conduction, to prevent the first switching element from being turned on and to thereby stop the switching operation. This allows for intermittent oscillation at light load, which has not been achieved by a two-switching-element self-excited-oscillation type switching power supply unit of the related art, by adding a small number of components.
According to preferred embodiments of the present invention, unlike the power supply unit disclosed in Japanese Patent Application Publication No. 11-187664, the second switching element is turned on before energy release from the secondary winding ends.
Thus, the power supply unit of preferred embodiments of the present invention can significantly reduce conduction loss and switching loss at light load and can perform a high-efficiency operation.
Also, the first switching control circuit is preferably configured as follows.
The first switching control circuit includes a first ON-time control circuit. When the output voltage reaches a predetermined value at light load, the first ON-time control circuit prevents a voltage generated across the first drive winding from turning on the first switching element to thereby stop oscillation. At rated or heavy load, the first ON-time control circuit allows a voltage generated across the first drive winding to turn on the first switching element, thereby controlling the ON-time of the first switching element.
At rated or heavy load, as for the first switching element to be turned on due to a voltage generated across the first drive winding, the first control transistor is driven to turn on the first switching element. This operation is analogous to the operation of the circuit of the related art.
At light load, a second ON-time control circuit changes the ON-time of the second switching element, to cause the energy release to continue at a turning-off timing of the second switching element. Further, after the energy release ends, no current flows through a rectifying element for the secondary-side winding, and when a reverse voltage is generated across the transformer, no current flows through the diode that is connected in parallel with the first switching element. At this point, causing the first switching element to be in the OFF state can turn the first switching circuit into a non-conduction state, thereby allowing the first switching control circuit to control an intermittent oscillation operation.
In contrast, with the circuit of the related art described above, when the second switching element is turned off and a reverse voltage is generated across the transformer, the first switching circuit cannot be brought into a non-conduction state since the diode, which is connected in parallel with the first switching element, goes into conduction. Thus, the circuit of the related art cannot control an intermittent oscillation operation.
An intermittent oscillation operation performs the following operation.
When the output voltage reaches a predetermined value due to an oscillation operation, before the first switching element is turned on due to a voltage generated across the first drive winding, the first control transistor is brought into conduction, to prevent the first switching element from being turned on and to thereby stop oscillation. The stopping of oscillation causes the output voltage to decrease, and when the output voltage reaches a predetermined value or less, a voltage is supplied to a control terminal of the first switching element via a start-up resistor, thereby starting oscillation. Repeating that operation cyclically provides an intermittent oscillation operation. The first switching control circuit includes the first ON-time control circuit that performs such operation at light load.
With the operation described above, a two switching-element self-excited-oscillation type switching power supply unit can perform an intermittent oscillation operation at light load.
In addition, a switching surge voltage is clamped during intermittent oscillation. Thus, low-voltage switching elements can be used to reduce conduction loss, which thus can achieve low loss.
Preferred embodiments of the present invention further provide the following configurations.
Preferably, the switching power supply unit according to a preferred embodiment of the present invention includes an output-voltage stabilization circuit. The output-voltage stabilization circuit detects the output voltage and, in response to the detected voltage, feeds back a signal for changing the impedance of the first impedance circuit by using a photocoupler, thereby stabilizing the output voltage.
Preferably, the output-voltage stabilization circuit is provided with a gain adjustment circuit. The photocoupler is constituted by a photodiode and is connected series with a resistor, and the gain adjustment circuit reduces the resistance of the resistor to increases a feedback gain of the photocoupler.
Preferably, the gain adjustment circuit is provided to allow for a significant reduction in output ripple voltage, which has been increased with a power supply unit of the related art.
During continuous oscillation of the switching power supply circuit, an output ripple voltage is small due to the absence of a non-oscillation period. Meanwhile, during an intermittent switching operation, the output voltage increases in an oscillation period and decreases in a non-oscillation period, which poses a problem in that ripple in the output voltage becomes large in an intermittent oscillation cycle.
Accordingly, in preferred embodiments of the present invention, a change in current flowing through the photodiode is increased during intermittent oscillation to cause the gain to increase. This can reduce and minimize the ripple voltage because of the improved sensitivity such that the ripple voltage is a very small fluctuation in output voltage.
Other preferred embodiments of the present invention further provide the following configurations.
Preferably, the switching power supply unit according to a preferred embodiment of the present invention may include a peak-current limit circuit. The peak-current limit circuit has a current detecting device that is connected in series with the first switching element. The current detecting device detects primary winding current that flows through the first switching element, and the peak-current limit circuit turns off the first switching element when the primary winding current reaches a current peak value.
With this arrangement, even when the primary winding current is increased, the peak current value is limited to prevent saturation in the transformer. Additionally, a reduction in current peak value allows for a reduction in switching loss when the first switching element is turned off. Further, the number of oscillation pulses in an oscillation period is increased to cause the cycle of an intermittent oscillation to be shortened, thereby allowing for a reduction in ripple voltage.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.